1. Field of the Invention
This invention is related to measurement of formation properties penetrated by a well borehole, and more particularly related to an electromagnetic wave propagation resistivity well logging system comprising multiple groups of electromagnetic transmitter-receiver arrays operating at three frequencies, with the lowest frequency being about 100 kHz. The system can be embodied to yield horizontal, vertical, azimuthally symmetrical, and azimuthally asymmetrical resistivity measurements with respect to the axis of the borehole.
2. Background of the Art
Earth formation resistivity is an important parameter in delineating hydrocarbon and saline water content within the pore space of the formation. Formations containing hydrocarbon in the pore space are typically more resistive than formations of the same lithology and porosity containing saline water in the pore space. Resistivity measurements are typically made by conveying an instrument or xe2x80x9ctoolxe2x80x9d along a well borehole penetrating the earth formation. The tool can be conveyed by a drill string thereby yielding a measure of resistivity while the borehole is being drilled. This type of system is commonly referred to as a logging-while-drilling (LWD) system. Alternately, the tool can be conveyed along the borehole by means of and electrical or fiber optic cable after drilling has been completed. This type of system is commonly referred to as a xe2x80x9cwirelinexe2x80x9d logging system.
Resistivity logging systems are typically electromagnetic, and comprise at least one transmitter and at least one receiver. Classes of resistivity logging systems are typically based upon the frequencies at which they operate. With respect to the lowest end of the frequency spectrum, there are electrode systems that operate in a range of around 1 kiloHertz (kHz). These systems rely upon the conduction of current through drilling fluid or drilling xe2x80x9cmudxe2x80x9d as part of the current flow path that also includes electrodes and the surrounding formation. Since the mud must be conductive, these systems will not operate in low conductivity mud systems such as oil based muds. With respect to the next frequency range, there are induction logging systems that operate in the range of 20 kHz. An induction log generates a magnetic field in the formation to produce secondary current flows within the formation. The secondary currents set up a secondary magnetic field, which induces electric signals in one or more receiver coils in proportion to the magnitude of the secondary current flow. The induced electric signal is directly proportional to the resistivity of surrounding formation. A measure of induced current can, therefore, be related to formation resistivity. Conductive mud is not required for operation. With respect to a much higher range of frequencies in the spectrum, there are electromagnetic wave propagation (EWP) systems that operate in the 500 kHz to 4 megaHertz (MHz) range. These systems typically use measured amplitude attenuation and phase shifts of induced currents to yield a measure of formation resistivity. Additional information on the basic concepts of prior art resistivity logging systems can be found in U.S. Pat. No. 5,081,419 to Meador et al.
Resistivity log measurements are affected by numerous perturbing factors in addition to the parameter of interest, namely the unperturbed or xe2x80x9ctruexe2x80x9d resistivity of the formation penetrated by the borehole. The perturbing factors include borehole size, borehole ruggosity, invasion of the drilling mud into formation in the immediate vicinity of the borehole, the resistivity of adjacent formations, dipping beds of contrasting resistivity, and the conductivity of borehole fluid. There are also design and operation properties of the logging system which affect the measure of true formation resistivity. These properties include operating frequency, spacings of transmitter-receiver groups, cross-talk between receivers, balance of transmitter-receiver groups, and the linearity of transmitter and receiver electronic circuits.
Transmitter-receiver operating frequency and spacing can be used to compensate for the adverse effects of borehole conditions and formation invasion. Holding all other variables constant, the depth of investigation of a given transmitter-receiver pair increases with increasing transmitter-receiver spacing. Again holding all other variables constant including transmitter-receiver spacing, the depth of investigation increases as operating frequencies decrease.
In order to determine and to compensate for perturbing effects of borehole conditions, invasion and adjacent bed boundaries, prior art resistivity logging systems have employed a plurality of transmitter-receiver groups operating at multiple frequencies. Furthermore, symmetrically arranged multiple transmitter-receiver pairs have been used to compensate for the effects of imbalance and cross-talk between transmitter-receiver pairs at differing spacings. Prior art EWP systems have, however, been limited to operating frequencies of about 200 kHz or greater. Lower operating frequencies such as 100 kHz at greater transmitter-receiver spacings will significantly increase depth of investigation of a logging system. This is highly desirable in minimizing the adverse depth of investigation related perturbations discussed previously. At operating frequencies around 100 kHz, however, phase and amplitude differences are relatively small and difficult to measure accurately. Furthermore, electronic circuits controlling the transmitters and receivers must be extremely linear for compensation of symmetric transmitter-receiver pairs to be effective. At increased spacing, low noise and high efficiency transmitters and receivers are needed.
Using tools with axial dipole antennas, one can obtain accurate formation resistivities in isotropic formations. In anisotropic or dipping-bed formations, measurements from such systems alone do not have all the information necessary to uniquely determine horizontal and vertical resistivities as well as the dip angle. Information from another source, in addition to the tool measurement, is required to completely determine the formation property. Drilling through anisotropic or dipping-bed formations is quite common in slant hole drilling operations.
In drilling a vertical borehole through formations that are essentially normal to the axis of the borehole, there is, therefore, no need for azimuthal xe2x80x9cfocusingxe2x80x9d of the resistivity measurement. In deviated borehole drilling operations, and especially in geosteering drilling of horizontal wells, azimuthal resistivity measurements are a necessity. Prior art resistivity logging systems using transmitter and receiver antenna lying in planes perpendicular to the axis of the logging tool are, as a group, not suited for azimuthal resistivity measurements.
The prior art discloses resistivity logging systems that use angularly skewed antenna in order to obtain measures of vertical resistivity, horizontal resistivity, and the angle of dipping anisotropic beds. Prior art azimuthally focused resistivity logging systems comprise antenna with sloped slot patterns, antenna shields, strip shields, and a variety of other specialized antenna elements. All known prior art systems require significant modification of basic antenna configurations found in xe2x80x9cconventionalxe2x80x9d resistivity logging systems in which all transmitter and receiver antenna are in planes perpendicular to the axis of the logging tool.
The present invention is an electromagnetic wave propagation (EWP) resistivity logging system that can be embodied as a logging-while-drilling system or as a wireline logging system. The tool portion of the system comprises a plurality of transmitter-receiver groups consisting of a total of eight transmitters and four receivers disposed axially and symmetrically about a reference point on the tool. A transmitter of a given transmitter-receiver group is activated, and induced signals are measured at preferably two axially spaced receivers within the group. A transmitter of a symmetrically opposing transmitter-receiver group is then activated and induced signals are measured in the two axially spaced receivers within that group. The system uses preferably three operating frequencies are used, with each transmitter-receiver group operating at two of the three operating frequencies. Each transmitter within a group is sequentially activated at one of two of the operating frequencies. Preferred operating frequencies are 2 MHz, 400 kHz and 100 kHz. Two transmitters disposed at a maximum spacing on opposing sides of the reference point are operated at 100 kHz and 400 kHz. The six remaining transmitters symmetrically disposed at three lesser spacings on opposing sides of the reference point are operated at 400 kHz and 2 MHz. Two receivers disposed on opposing sides of the reference point are operated at 100 kHz and 400 kHz. The remaining two receivers disposed on opposing sides of the reference point at a lesser spacing are operated at 400 kHz and 2 MHz. The transmitters and receivers are powered and operated by low noise, highly linear electronic circuitry within the tool. The tool is conveyed along a borehole, penetrating an earth formation, by a conveyance system which includes a surface conveyance unit and a member which extends from the tool to a surface conveyance unit. If the system is embodied as a LWD system, the surface conveyance unit is a drilling rig and the member is a drill pipe string. If the system is embodied as a wireline logging system, the surface conveyance unit is a wireline draw works and the member is a wireline logging cable. Other conveyance systems can be used to convey the tool along the borehole. These conveyance systems include a slick line, coiled tubing, and coiled tubing with a conductor embedded within the tubing. The surface conveyance unit also cooperates with surface equipment which powers and controls the operation of the tool. The surface equipment also preferably cooperates with a computer or processor which is programmed to record responses from the four receivers, compute amplitude attenuation and phase shift, and transform these computations into parameters of interest, such as true formation resistivity.
Each transmitter and receiver element comprises a plurality of axial slots fabricated within the wall of the tubular pressure housing member of the tool. Alternately, the transmitter and receiver elements can be fabricated as separate units and subsequently integrated into the housing member. Each slot is defined by adjacent lands with an outer radius essentially equal to the outer radius of the tool housing, and a surface of lesser radius. An essentially circular antenna, in a plane perpendicular to the axis of the tubular housing member, traverses each slot and traverses each land through a radial antenna pathway therein. Magnetic reluctance material such as ferrite is positioned within the slots on the surface of lesser radius. This increases antenna efficiency by increasing gain for a given power level. In an alternate embodiment of the invention, the antenna pathways of the low frequency transmitters and receivers are coated with highly conducting material to increase antenna efficiency. In yet another embodiment, the interiors of the slots are also coated with the highly conducting material still further increasing antenna efficiency.
By using an operating frequency of 100 kHz or lower along with higher operating frequencies, and by using the previously discussed transmitter-receiver group spacings, depth of investigation of the tool is greater than any known prior art EWP resistivity logging system. This increased depth of investigation is advantageous in obtaining true formation resistivity in highly invaded formations. The increased depth of investigation is also advantageous when the tool is embodied as a geosteering system which measures the distance between the tool and adjacent formations of contrasting resistivity.
The position of the magnetic reluctance material can be varied within the slots to yield measurements that would be obtained if the antenna were skewed. As an example, magnetic reluctance material inserts of a given dimension can be arranged in the axial slots so that their mid points fall in a plane which is not perpendicular to the axis of the logging tool. This antenna element will then respond to resistivity, as would a skewed antenna coil, even though the actual antenna wire remains in a plane normal to the axis of the tool. Responses from these xe2x80x9cmodifiedxe2x80x9d antenna elements are combined with responses from xe2x80x9cunmodifiedxe2x80x9d antenna elements to yield vertical resistivity, horizontal resistivity, and the angle of dipping anisotrophic beds.
Azimuthally sensitive resistivity measurements can be obtained by varying the dimensions of the magnetic reluctance material inserts while holding the mid points of the inserts in a plane normal to the axis of the logging tool. This is because the sensitivity of a given azimuthal arc of antenna element is related to the amount of magnetic reluctance material within an axial slot defining the arc. Azimuthal focusing is, therefore, obtained without the need to modify slot geometry, or to modify the antenna position within the element, or without the use of a masks or shields of any type.